For adjustment of the desired operating voltage, fuel cell units are arranged one on top of the other in the required number in order to form a fuel cell stack. To prevent an electrical short-circuit, the housings of fuel cell units arranged consecutively in the fuel cell stack are electrically insulated from one another. It is additionally necessary to separate the fuel gas ducts of the fuel cell units from the oxidising agent chambers of the fuel cell units in a gastight manner and the oxidising agent ducts of the fuel cell stack from the fuel gas chambers of the fuel cell units in a gastight manner.
In known fuel cell stacks, sealing and insulation elements made of glass solder or ceramic sealing materials are used in order to obtain the necessary electrical insulation effect and the necessary sealing effect.
In the case of some of the sealing materials usually used, the electrical resistance is no longer sufficiently high at the operating temperature of a high-temperature fuel cell unit (in the range of approximately 800° C. to approximately 900° C.) to attain a satisfactory insulation effect. Moreover, some of the sealing materials usually used have only a low stability with respect to the temperature changes (between operating and resting phases) that frequently arise with a high-temperature fuel cell unit.